It is known to build physical three-dimensional models using additive manufacturing technology. Typically, a virtual design of the three-dimensional model, e.g. represented through computer-aided design (CAD) software or the like, is transformed into a plurality of thin (quasi-two-dimensional) cross-sectional layers which are built on one another in succession.
In some circumstances, it may be desirable to replace the interior of a virtual solid three-dimensional model (e.g. a CAD model, in any of a number of possible formats) with a void or an internal framework. A model with an internal void may resemble a shell of the original model. An internal framework may comprise an array of support struts or a microstructure such as an open-celled foam or the like. There are a number of reasons for replacing the solid interior of an original model. One reason may be to use less material in the creation of a physical model (e.g. by a three-dimensional printer). Another reason may be to create a lighter model whilst maintaining appropriate stiffness or rigidity. A further reason may be to enhance the model's physical properties in some way, e.g. to prevent warping or the like. For example, an internal microstructure may give an object different electromagnetic properties, which may be of interest in the field of radar technology.
The conventional approach to hollowing out a solid CAD object starts with a CAD representation of the object, which is a collection of vector-based representations of graphical entities corresponding to the different outer surfaces of the object. To achieve hollowing out, the conventional approach moves the outer surfaces inwards by a fixed amount (typically specified by the user). The set of surfaces spawned by this movement are designated as internal surfaces, whereby the original (external) and spawned (internal) surfaces are used to define the exterior and interior of the hollowed out model. This process is sometimes called shelling or offsetting.
Difficulties can arise with this conventional approach because displacing outer surfaces inwards can cause the formation of new intersections or result in surfaces not meeting at edges. It is possible to handle these difficulties for relatively simple objects, although inconvenient if it must be done manually. However, for more complex objects, such as those requiring a microstructure to be formed in the interior of the object, these difficulties present significant problems.
One known attempt to introduce a microstructure into a three-dimensional CAD object involved merging a CAD description of a microstructure with the internal surface(s) of the hollowed out model spawned by the shelling process described above. This technique is difficult and unreliable, e.g. because the intersections between the microstructure and the internal surface(s) of the hollowed out model need to be recomputed in order to create a new closed internal surface. As the complexity in the geometry of the microstructure increases, this process becomes non-trivial and far from robust and often results in undesirable sharp edges where the microstructure and the CAD model interface.